Alarm transmission line security system utilizing pseudo random encoding

ABSTRACT

An alarm line security system wherein areas to be protected each include an alarm source having an intrusion detector and an alarm source monitoring circuit including a code generator for generating a code comprised of a pseudo-random sequence of bits for transmission over an alarm transmission line to alarm monitoring circuits at a central monitoring area, and a code complementor circuit controlled by the intrusion detector for modifying the generated code prior to transmission to the central monitoring area whenever an unauthorized entry is detected. A multiplexer circuit interposed between the alarm source monitoring circuits for all of the protected areas and the transmission line enables transmission of one or more of the bits which comprise the pseudo-random codes generated at each alarm source to be transmitted over a common transmission line. The alarm monitoring circuits at the central monitoring area include a reference code generator which generates the same pseudo-random code as the code generators associated with the alarm sources, and a code comparator circuit which compares the code received from each source to the reference code and provides an alarm indication whenever codes from one or more of the alarm sources differ with the reference code.

United States Patent [191 Donovan et al.

[54] ALARM TRANSMISSION LINE SECURITY SYSTEM UTILIZING PSEUDO RANDOMENCODING [75] Inventors: John C. Donovan, Whitefish Bay; RameshKrishnaiyer, Milwaukee; Frank J. Esser, Wauwatosa, all of Wis.

[73] Assignee: Johnson Service Company, Milwaukee, Wis.

[22] Filed: Oct. 28, 1971 [21] Appl. No.: 193,450

[52] US. Cl. ..340/l64 R, 340/274 [51] Int. Cl... .4 ..G08b 13/08 [58]Field or Search ..340/l64, 167, 147, 340/274 [56] References CitedUNITED STATES PATENTS 3,611,293 10/1971 Constable ..340/l49 A 3,654,6044/1972 Crafton ..340/147 R Primary Examiner-Donald J. YuskoAttorney-John A. Dienner, Arthur C. Johnson, John A. Dienner,.lr. et al.

sac

[57] ABSTRACT An alarm line security system wherein areas to beprotected each include an alarm source having an intrusion detector andan alarm source monitoring circuit including a code generator forgenerating a code comprised of a pseudo-random sequence of bits fortransmission over an alarm transmission line to alarm monitoringcircuits at a central monitoring area, and a code complementor circuitcontrolled by the intrusion detector for modifying the generated codeprior to transmission to the central monitoring area whenever anunauthorized entry is detected. A multiplexer circuit interposed betweenthe alarm source monitoring circuits for all of the protected areas andthe transmission line enables transmission of one or more of the bitswhich comprise the pseudo-random codes generated at each alarm source tobe transmitted over a common transmission line. The alarm monitoringcircuits at the central monitoring area include a reference codegenerator which generates the same pseudo-random code as the codegenerators associated with the alarm sources, and a code comparatorcircuit which compares the code received from each source to thereference code and provides an alarm indication whenever codes from oneor more of the alarm sources differ with the reference code.

22 Claims, 4 Drawing Figures 4 L41? M LINE MON! TOR/N6 w @I W CLOCK LINEcLacx LINE PATENTED MY 2 2 I975 SHEET 3 BF 4 INVENTORS JOHN c. DONOVANREMESH KRISHNA/YER FRANK J. ESSER BY f w g/MIA bw QNSSQQ QRE w th MQQEW2 QQREGRMQ MQQQ Sbtk a QEEmEQ 33 :ER

w QB 35% 38 BE A TTYS ALARM TRANSMISSION LINE SECURITY SYSTEM UTILIZINGPSEIJDO RANDOM ENCODING BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to security systems and moreparticularly to a system for transmitting alarm signals from a protectedarea to a central monitor via a communication line.

2. Description of the Prior Art In known security systems it has beendifficult heretofore to reliably provide indications at a centrallocation of an unauthorized entry to an area being protected. Typically,the protected area is a room containing an intrusion detection systemwhich is capable of generating an alarm signal whenever an intrusion hasbeen sensed. Such alarm signal, in many systems, must be transmitted viaassociated lines to a central monitor.

In an intrusion detection system, the transmission lines between theprotected area and the monitoring area are typically the most exposedand the most accessible part of the security system. A variety ofapproaches have been employed in the past in an attempt to ensure thattampering with these lines has not occurred.

One type of known system relies on the presence of a DC current on theinterconnecting transmission line. Until recently, closed DC loopcircuits which are sensitive to changes in excess of 40 percent of linecurrent have been used as the industry standard. Further systems thatprovide AC pulses superimposed on a standard UL approved closed loop DCcircuit are also presently available. Although these circuits are moredifficult to compromise than a circuit with a sensitivity to a 40percent variation in signal, they likewise can be compromised by voltageas well as resistance substitution.

A more recently developed system is based on the principle of a balancedbridge circuit. In its simplest form, this system merely employs asensitive indicator in a carefully balanced bridge circuit. The systemcomplexity is enhanced by using multi-level DC excitation and/ormulti-level, multi-frequency AC excitation together with matchedterminating modules. One of these matched modules is located in theprotected area while the other is located with the bridge at themonitoring end of the line. These modules are typically complexcombinations of linear and nonlinear, passive and active elements.Properly applied, such balanced bridge systems provide the greatestdegree of line security in commercially made line security equipmentwhich are available at the present time. These systems can be adjustedto an extremely high level of sensitivity. At the high sensitivitysetting, however, frequent false alarms are generated by variations inthe monitoring signal caused by environmental changes, component aging,minor noise sources, and the like. To prevent such false alarms, thesystem sensitivity is reduced with an attendant reduction in thesecurity level.

Other techniques include the use of highly complicated cryptographictechniques that tend to remain tamperproof for an extended time period.However, the complexity of operation and maintenance as well as theinitial cost along with the need to operate on relatively narrowbandwidth transmission lines dictate against the use of such systems.

The problems with these various systems point to the need for a simple,inexpensive, fail-safe system for supervision of transmission lineswhich extend from the alarm generating apparatus in a protected area toa control monitor.

In security systems, it is common to transmit alarm signals from aplurality of remote intrusion detection systems to a central alarmmonitoring area over individual wires. If the transmission distances aregreat, the cost of the required individual pairs of transmission wiresis significant. If there are several protected areas in close proximityto one another, but relatively remote from the monitoring area, it wouldseem reasonable to use a system including a multiplexing arrangement tocompress the several channels of alarm information into a form which canbe carried on a single pair or a relatively few number of transmissionwires.

The art of multiplexing, such as time division multiplexing, is welldeveloped in other areas. Typical examples include the areas of voiceand digital data communications as well as the area of industrialinstrumentation and control. In time division multiplexing, thecondition of one channel is sampled for a short time interval andtransmitted to the central monitoring area. Then the condition of thenext channel is sampled and transmitted in the next interval, etc. Atthe receiving end, the samples of each channel are separated, and theindividual signals are reconstructed.

Despite the level of sophistication to which the art of multiplexing hasrisen in other application areas, it has not found much acceptance inthe security area. There is a prevailing feeling that a necessarycondition for maximum security requires the connection of a permanent,hard-wired transmission line between the intrusion detector in theprotected area and the alarm unit in the monitoring area. Thus, any timedivision multiplexing system which inherently results in a briefinterruption between the detector and the alarm unit to provide a timeinterval for interleaving of the signals of several channels has notbeen well accepted. A part of this disfavor results from the likelihoodthat a malfunction in the multiplexer switching circuitry will result inan open in the alarm line. There is also the fear that an alarm mayoccur on a given channel between the two instants at which a givenchannel is being sampled.

The present invention provides a security alarm multiplexing systemwhich employs an alarm line supervising technique wherein the functionof providing coded line security signals for indicating alarminformation for each of a plurality of protected areas and the functionof multiplexing of the coded signals for transmission to centralizedmonitoring equipment are integral with one another.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide an alarm line security system which realizes a maximum level ofsecurity for transmission lines which are used to carry alarminformation from remote areas protected by intrusion detection systemsto a central alarm monitoring area. This system could, of course, beemployed in applications other than intrusion detection systems whichrequire secure transmission lines between two physically separatedlocations.

lt is another object of this invention to provide a line security systemcapable of protecting low grade telephone lines, low grade twisted pairsor lines installed especially for security monitoring purposes.

It is a further object of the invention to provide an alarm linesecurity system in which the alarm signal is inherent in the linemonitoring signal and is indistinguishable therefrom, whereby a personwho is attempting to defeat the system by monitoring an alarmtransmission line would not know at which point his entry into aprotected area will result in the generation of an alarm signal.

It is yet another object of the invention to provide a system forsequentially relaying the alarm signals from a plurality of remoteintrusion detection systems to a central alarm monitoring area over asingle pair of wires, which wire pair is secured from tampering by thetransmission of a known selected sequence code bits from the remotelocations to the central alarm monitoring area.

In one embodiment, the alarm line security system provided by thepresent invention includes a plurality of alarm sources which includeintrusion detectors located in areas to be protected, each of whichdetectors provides an alarm indication responsive to the detection of anunauthorized entry of one of the protected areas. Each alarm source isconnected to a separate alarm source monitoring circuit which includes acode generator which generates a selected sequence of code bits fortransmission to alarm line monitoring circuits at a central monitoringlocation. The coded sequence of bits from each alarm source monitoringcircuit are multiplexed for transmission to the central monitoringlocation over a single pair of transmission alarm lines.

Furthermore, each alarm source includes an associated alarm sourcemonitoring circuit having a code generator and means responsive to thedetection of an unauthorized entry, to modify code bits output by thecode generator to indicate an alarm.

The alarm line monitoring circuits at the central monitoring area alsoinclude a reference code generator which generates a sequence of codebits which is identical to that provided by the code generatorsassociated with each alarm source.

The alarm line monitoring circuit also includes a code comparatorcircuit which accepts the coded bits transmitted to the centralmonitoring area over the alarm line from each alarm source and comparessuch bits with the reference code bits generated by the reference codegenerator at the central monitoring location. Under normal conditions,(i.e., when all protected areas are secure), the sequence of bitsreceived from the remote alarm sources will be identical with thesequence of bits provided by the reference code generator. However,whenever an alarm indication is provided by one or more of the alarmsources, the code transmitted from such sources will differ from thereference code. With the detection of a difference in one or more bitsby the code comparator circuit, an alarm output signal is provided toenable an alarm to be registered at the central monitoring area.

A multiplexing circuit, interposed between the outputs of the alarmsource monitoring circuits at each protected area and the alarmtransmission line, serves to enable the transmission of one or more ofthe bits of the selected sequence of bits provided by each alarm sourcemonitoring circuits to the central monitoring location. By way ofexample, for a given cycle the sequence of bits transmitted to thecentral monitoring area may include the first bit of the code sequenceprovided by a code generator associated with an alarm source at a firstprotected area, the second bit provided by a code generator associatedwith an alarm source at a second protected area, etc., with the last bitbeing provided by a code generator associated with the alarm source atthe N" protected area.

In accordance with the feature of the invention, with the sequence ofbits transmitted from a given number N of protected areas as describedabove, the resulting code received at the central monitoring locationwill be identical to the sequence of bits generated by code generatorsat each remote alarm source and also to the reference code sequencegenerated at the central monitoring location.

Thus, a comparison of the multiplexed code received from the remoteequipment and the reference code generated in the central monitoringfacility simultaneously provides for the security of the branch lineswhich interconnect the individual remote alarm sources with themultiplexer and the main trunk line which connects the multiplexer tothe central monitoring area.

The alarm line monitoring circuits include a demultiplexer circuit whichseparates the time interleaved code bits transmitted from the alarmsources via the multiplexer to permit identification of the alarm sourcewhich is providing an alarm indication and to enable registration of thealarm indication at the central monitoring area.

In accordance with a further feature of the invention, the operation ofthe multiplexer, the demultiplexer and the code generators, includingthe reference code generator, are synchronized by sync pulses providedby a single sync pulse generator at the central monitoring location. Inaddition, means are provided for resynchronizing the code generatorcircuits and the multiplexer and demultiplexer circuits in the eventthat one of these circuits fails to receive one of the sync pulses or inthe event that the circuit accepts a noise spike as a sync pulse. Suchresynchronization is provided after an alarm indication is registeredbut before an investigation.

It is further noted that the alarm signal is inherent in the linemonitoring codes generated by the code generators associated with thealarm sources, such that the alarm indication is indistinguishable fromthe line monitoring codes. Therefore, a person attempting unauthorizedentry or tampering with the alarm transmission lines would be unawarethat the code has been changed due to the random nature of the linemonitoring code provided by the code generators.

Other objects and advantages of the invention will become apparent fromthe following detailed description of the invention which makesreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an alarmtransmission line security system provided by the present invention;

FIG. 2 is a schematic representation of an alarm source monitoringcircuit of the system shown in FIG.

FIG. 3 is a schematic block diagram of a multiplexer circuit of thesystem shown in FIG. 1; and

FIG. 4 is a schematic representation of an alarm line monitoring circuitof the system shown in FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT General Description Referring tothe block diagram of an exemplary illustration of a multiplexed secureline alarm transmission system shown in FIG. 1, the system comprises aplurality of alarm sources, including alarm sources 11-14 shown in blockform in FIG. 1, for providing alarm indications for transmission toalarm line monitoring equipment 31 at a central monitoring location viaassociated alarm source monitoring circuits 21-24, a multiplexer circuit28 and a transmission line 30. Each alarm source, such as alarm source11 is located in an area or zone to be protected. Typically, theprotected area may be a room.

Each of the alarm sources 11-14, such as alarm source 11, controls anassociated alarm source moni toring circuit such as circuit 21associated with alarm source 11. Preferably, the alarm source monitoringcircuit is located within the area protected by an associated entrydevice. Monitoring circuit 21 includes a code generator 25, an alarmconverter 26 and a code complementor circuit 27.

The alarm source monitoring circuits 22-24 which are associated with thealarm sources 12-14 are identical to alarm monitoring circuit 21, andeach include an individual alarm converter, code generator, and codecomplementor circuit. Under normal conditions, that is when no alarm isprovided by alarm source 11, for example, the code generator 25associated with alarm source 1 1 generates a predetermined series ofdata bits or multi-bit code words which are transmitted to the alarmline monitoring equipment via multiplexer 28 and transmission line 30.The alarm line monitoring equipment 31 includes a reference codegenerator 32 which is identical with each of the code generators such asthe code generator 25 at the remote station 11. A code comparator 33which comprises the alarm monitoring equipment 31 compares the referencecodes provided at the central location by the reference code generator32 with the information code being generated by each of the codegenerators at the remote locations 11-14. Whenever the codes areidentical, the code comparator provides an output indicating that noalarm is being sent from any one of the monitoring positions. In theevent of an alarm at zone 1, for example, the alarm source 11 at zone 1enables the code complementor circuit 27 of the alarm source monitoringcircuit 21 associated with alarm source 11 to complement the code beingsent to the central location from remote zone 1. Consequently, when thecomplemented code sent from zone 1 is received the difference in thecodes will be indicated by the code comparator 33 at the centralmonitoring station. The comparator will provide a different output whichindicates an alarm condition is to be registered for zone 1. The centralmonitoring includes suitable alarm registers 36 which indicate which ofthe remote positions has registered an alarm, zone 1 in the presentexample.

The alarm transmitting system employs time division multiplexing whichenables the line security codes generated by each of the alarm codegenerators associated with alarm sources 11-14 to be transmitted to thealarm monitoring equipment 31 over the same transmission line 30.Accordingly, the alarm source monitoring equipment includes amultiplexer circuit 28 at one of the alarm monitoring locations forapplying the signals provided by the code generators which comprise thealarm source monitoring circuits 21-24 to the transmission line 30 atseparate time intervals or time slots, and the alarm line monitoringcircuits 31 include a demultiplexer circuit 35 which separates the timeinterleaved signals from the time slots. As will be shown hereinafter,the output code produced as a result of multiplexing the output of allof the alarm source monitoring circuits is the same code as would beprovided by a single alarm source monitoring circuit. Thus, an alarmcondition from any one of the alarm sources 11-14 can be detected witheach scan. Synchronization of the operation of the code generators, themultiplexer circuit 28 and the demultiplexer circuit 35 is provided bysync pluses generated by a sync pulse generator circuit 34 whichcomprises the alarm line monitoring circuits 31. The sync pulsesprovided by sync pulse generator 34 are transmitted to the remote alarmsources 11-14 over lines 30E, 30 and 3118. Transmission over line 30 isby half duplex transmission terminals 30C and 30R which isbi-directional. Sync pulse transmission is unidirectional from the alarmline monitoring circuits 31 to the remote alarm sources.

Detailed Description Referring to FIG. 2, there is shown a schematicblock diagram of the alarm source 11 and the associated sourcemonitoring circuit 21.

In one application of the invention to an unauthorized entry detectionsystem, the alarm sources 11-14 comprise intrusion detector means, suchas an entry switch for indicating an unauthorized entry of the protectedarea. However, other types of alarm indications, such as the presence ofa fire may be conveyed from protected areas to a central monitoringlocation using the apparatus provided by the present invention. In theembodiment illustrated in FIG. 2, the alarm source 11 is shown toinclude an alarm switch 15 having normally closed contacts 15a.

The alarm contacts 15a are connected over a pair of conductors 16, 17 tothe alarm converter circuit 26 which converts the open or closed contactposition to a logic level output signal which, for example, may indicatean alarm condition as a logic 1 level and the absence of an alarm as alogic 0 level. The alarm converter in this example merely connects line17 to ground and line 16 to line 26a. However if the alarm source is nota pair of contacts, but, for example, a voltage input the alarmconverter functions to transduce such input voltage to a signalcompatible to the code complementer 27 input. The output of the alarmconverter circuit is extended via conductor 26a to an input of the codecomplementer circuit 27. A second input to the code complementer circuit27 is connected to the output of the pseudo-random code generatorcircuit 25.

The code generator circuit 25 associated with alarm source monitoringcircuit 21 consists of a four stage shift register 45 having feedbackconnections over conductors 46 and 47 from the first and fourth stagesrespectively connected through an exclusive OR circuit 48 to the inputof the first stage. The exclusive OR circuit 48 provides a logic 0output whenever the two inputs to the exclusive OR circuit are the samelogic level, and provides a logic 1 output whenever the inputs aredifferent logic levels. Code generator circuit 25 preferably includes atleast a 16 stage register and may be as much as a 32 stage registercapable of generating a pseudo-random sequence of bits, the length ofthe sequence being given by the relationship 2 l where N is the numberof stages which comprise the shift register of the code generatorcircuit 25. In the present example, for convenience only, a four stageregister 45 is used, fifteen bits being provided. Thus, it is apparentthat the shift register 45 may comprise any number of stages, and thatcorrespondingly, a sequence of bits given by the relationship 2,, I willbe provided. The number of bits desired would be dependent upon theamount of security required and the economics of the system.

To illustrate the operation of the code generator it is assumed thatinitially all stages are loaded with logic 1 levels and that theregister 45 is thereinafter cycled under the control of interrogate orsync pulses provided by a sync pulse generator 34 of the alarm linemonitoring equipment 31, transmitted via cable 30 to the remote alarmzone equipment 21-24. The sequence of words given in Table 1 will appearin the stages of the shift register 45.

TABLE I Register Stage I 2 3 4 (Initial) l l l 1 Clock Pulse 1 l l l 2 l0 l l 3 0 l 0 l 4 l 0 l 0 5 l l 0 l 6 0 l l 0 7 0 0 l l 8 l 0 0 l 9 0 lO 0 10 0 0 l 0 11 0 0 0 1 12 l O 0 0 13 1 l 0 0 14 1 l l 0 15 l l l ISince initially, stages 1 and 4 both contain binary ls, the exclusive ORcircuit 48 provides a logic 0 output which is gated into the first stageof the shift register 45 with receipt of the first sync pulse as can beseen in step 1. The sync pulse also shifts the logic ls from stages 1-3to stages 2-4, respectively. When the second clock pulse is received,the outputs of stages 1 and 4 are different, and accordingly theexclusive OR circuit 48 will provide a logic I output which will begated into first stage of the shift register 45 as the bits in stages 1through 3 are gated into stages 2-4, respectively. In a similar fashionfor sync pulses 3-14, outputs fed back to the input of the first stagecause the sequence of words given in Table l to be generated.

The sequence given in Table l repeats after the 15 sync pulse. Thus, itis seen that 15 pseudo-random words are provided when the number ofstages of the shift register is equal to four. The total number ofcombinations of N bits is 2, so only one combination is missing. Themissing multibit word is 0000 which can be seen to repeat itself or havea trivial cycle of one in the code generator circuit.

As has been indicated in the foregoing, the code generators whichcomprise the alarm source monitoring circuits 21-24 are identical.Moreover, the reference code generator 32, shown in FIG. 1, associatedwith alarm line monitoring equipment 31 at the central monitoringposition is also identical to the alarm source code generator, such ascode generator 25 of circuit 21. Accordingly, the feedback connectionsof the registers which comprise these code generators are the same asthose shown for register 45 which comprises code generator 25. The codegenerators are, so to speak, programmed identically and furthermore theyoperate in synchronism under the control of clock or sync pulsegenerator 34 associated with the alarm line monitoring equipment 31 atthe central locations.

The code generators which comprise alarm source monitoring circuits21-24 and code generator 32 of the alarm line monitoring circuit 31 eachproduces the pseudo-random code given in Table I. Under the control ofthe multiplexer, one of the code bits, or one multi-bit word generatedby each code generator associated with the alarm sources 11-14 aretransmitted in sequence via transmission line 30 to the alarm linemonitoring equipment 31 shown in FIG. 1. The reference code generator 32associated with the alarm line monitoring equipment 31 simultaneouslyprovides an identical sequence of bits or multi-bit words and the bitsthus provided are compared by the logic comparator circuit 33. If thebits are identical, the protected zones and the transmission lines aretaken to be secure. If the bits are different, the comparator circuit 33provides an output signal which registers an alarm in the alarm register26.

In accordance with one embodiment, alarm signals provided by the alarmsource monitoring circuits 21-24 are coded so that the alarm signals areindistinguishable from the line monitoring code provided by the remotecode generators, such as code generator 25 Such coding is accomplishedby the code complementor circuit 27 which, in turn, is controlled by thealarm converter 26 responsive to an alarm from an associated alarmsource 11 at protected area 1. The code converter 27 complements theoutputs provided by the code generator 25 so that the alarm condition isdetectable at the monitoring end by the alarm line monitoring equipment31 but is indistinguishable from other codes on the line 30 to a personattempting to tamper with the line. An alarm is defined as a logic 1level and the absence of an alarm as a logic level 0. The complementingoperation corresponds to an exclusive OR operation, as shown in TableII.

TABLE II Code generator bits 0 l 0 l Alarm bits 0 0 l l Output bits 0 ll 0 As can be seen in FIG. 2, for alarm source monitoring circuit 21,the output of the alarm converter circuit 26 is connected to one input50 of the exclusive OR circuit 49 which comprises the code complementorcircuit 27 and the output of the code generator circuit 25 is connectedto a second input 51 of the exclusive OR circuit 49 which comprises thecode complementor 27. Thus, in accordance with the truth table given inTable II, whenever the alarm bit is a logic 0 level indicating theabsence of an alarm, the code generated by the code generator 25 will bepassed to the output of the code complementor 27 to the multiplexer 28.On the other hand, whenever the alarm bit is a logic 1 level indicatingan alarm condition, the output of the code generator 25 will be invertedand the inverted output will be passed to the multiplexer 28.

MULTIPLEXER CIRCUITS Referring again to FIG. 1, the output of each ofthe alarm source monitoring circuits 21-24 are connected over branchlines 52-55 to individual inputs of a time division multiplexer circuit28. Preferably, the multiplexer circuit 28 is located within the areaprotected by one of the alarm sources, such as area one which isprotected alarm source 11. As has been indicated, each code generatorprovides the same pseudo-random codesequence as shown in Table III.

TABLE Reference Code Generator Remote generator-source 1 Remotegenerator-source 2 Remote generator-source 3 Remote generator-source 4Remote generator-source 5 Remote generator-source N1 Remotegenerator-source N Time Period The multiplexer 28 transmits a bit .froma given remote generator during a given time period, interleaving theinformation in time such that during a first time interval, informationfrom the code generator 25 associated with the alarm source 11 in area 1is transmitted over the main trunk alarm line 30 to the centralmonitoring equipment 31. During a second time interval, the informationfrom the code generator associated with alarm source 12 in area 2 istransmitted over alarm line 30 to the central monitoring equipment 31,etc., until the N" information bit provided by the code generatorassociated with alarm source 14 in area N is returned to the centralmonitoring equipment 31. The bits which are transmitted from eachlocation are circled in Table III. Then the scan repeats with the N 1bit produced by code generator 25 associated with alarm source 1 beingreturned to the alarm line monitoring equipment 31.

It is pointed out that the resulting code sequence transmitted to thecentral monitoring circuits 31 during time intervals 1, 2, 3, etc., (asshown in Table III) specificallythose bits circled in Table III, isidentical to the code sequence produced by any individual remote codegenerator and to the code produced by the reference code generator inthe central monitoring area.

Thus, a comparison of the multiplexed code received from the remoteequipment and the reference code generated in the central monitoringfacility simultaneously provides for the security of the branch lines52-55 connected from the individual remote alarm monitoring circuits21-24 to the multiplexer 28 as well as the main trunk line 30 connectedfrom the multiplexer 28 to the central monitoring area 31.

The security of each branch line 52-55 is checked once in the same timethat the security of the main trunk line 30 is checked N times.

As has been pointed out, the closure of the alarm contacts of a givenalarm detector such as contacts a associated with alarm source 11,enables the code complementing circuit 27 which inverts the code issuingfrom the code generator for that zone. Thus, when confirming the codefor zone 2, for example, an alarm would indicate that zone 2 had beenviolated or that the branch cable 2 zone has been tampered with. Incontrast, failure of a number of the code bits to coincide on a givencycle would suggest the main trunk cable 30 had been tampered with.

Through the provision of a latching circuit or a time delay, an alarmindication provided by an alarm source can be maintained for a timesufficiently long that the alarm will be transmitted to the alarmmonitoring circuits 31 by at least two cycles of complemented codes.That is, an alarm will be held on for a period of time sufficientlygreater than the time required to scan all remote points at least oncesuch that the alarm indication will be transmitted to the alarm linemonitoring circuits at least twice.

The multiplexer circuit 28 shown in block diagram form in FIG. 3includes a ring counter 60 having N stages. The multiplexer circuit 28further includes a set of output gates, such as gates 61-64 one for eachstage of the ring counter 60. The ring counter 60 provides an individualstage for each alarm source, such as alarm sources 11-14. The last stageN of the ring counter 60 is connected over lead 66 to the input stage ofthe ring counter 60.

The outputs of the stages 1, 2, 3, 4, N of the ring counter 60 areindividually connected to inputs of the AND gates 61-64, respectively.The AND gates 61-64 are individually associated with the alarm sourcemonitoring circuits 21-24, (FIG. 1) respectively, and are operable whenenabled to selectively extend the coded output signals provided by thealarm monitoring circuits 21-24 to the central monitoring equipment 31.Thus, a second input of each AND gate 61-64 is individually connected tothe output of the alarm monitoring circuits 21-24 respectively. Theoutputs of the AND gates 61-64 are connected to OR circuit 110, whoseoutput is fed to AND gate 112. AND gate 112 is gated by clock drivenmonostable oscillator circuit 114 to transmit over half duplex terminals30R and 30C (FIG. 1). One commercially available monostable oscillatorsuitable for the application is the type SN 74121.

One of the stages of the ring counter 60 contains a logic 1 level bit,and the remaining stages contain logic 0 level bits. The logic 1 levelis shifted through the stages of the ring counter 60 by the sync pulsesprovided by the central monitoring equipment 31 for driving thepseudo-random code generators associated with the alarm sources, such ascode generator 25 which comprises alarm monitoring circuit 21 associatedwith alarm source 11. The sync pulses received on line 30 B are extendedto the ring counter 60 of the multiplexer circuit 28 via sync pulse line67. The logic 1 level provides an enabling signal for the AND gate thatis connected to the output of the stage which contains the logic 1level. Thus, for example, if the logic 1 level is in the first stage ofthe ring counter 60, an enabling signal will be extended to one of theinputs of AND gate 61 which is associated with alarm monitoring circuit21. Thus, if alarm monitoring circuit 21 is providing a logic 1 output,AND gate 61 will be enabled and the logic 1 level will be passed to line30 for transmission to the central monitoring equipment 31. If, on theother hand, the alarm monitoring circuit 21 is providing a logic 0output, gate 61 will not be enabled and will provide a logic 0 output tothe line 30. As the logic 1 level is shifted through the ring counter60, the outputs of the alarm monitoring circuits 21-24 are sequentiallyconnected to the transmission line 30 over respective gates 61-64.

Accordingly, as each AND gate is enabled in sequence, such gate willprovide a logic 1 output whenever the associated line supervisioncircuit is providing a logic 1 output and will provide a logic outputwhenever the associated line monitoring circuit is providing a logic 0output.

Only one of the AND gates 61-64 will be enabled at a given time, andaccordingly, the output provided by only one of the line supervisioncircuits 21-24 will be passed by OR circuit 110 to the centralmonitoring equipment 31 during a given time period.

MULTIPLE BITS FROM EACH ZONE In the foregoing description, it wasassumed that the multiplexer circuit 28 was operative to effect thesequential transmission of one bit from each of the alarm monitoringcircuits 21-24. However to minimize nuisance alarms which may be causedby the error in one bit of the pseudo-random sequence of bits providedby a given line monitoring circuit it may be desirable to transmit amulti-bit code comprising a plurality of bits from each location. Forexample, four bits can be transmitted from each remote location byconnecting a divide-by-four pulse counter circuit 68 in place of link 69in series with the sync pulse line 67 and the ring counter 60.Accordingly, the ring counter 60 of the multiplexer 28 will be steppedonce for each four bits read out with every four advance cycles of thecode generators, and accordingly, four code bits will be extended to theAND gates 61-64 for each cycle of multiplexer circuit 28.

ALARM MONITORING EQUIPMENT Referring to FIG. 4, the code bitstransmitted from the remote station by the multiplexer 28 and cable 30are extended to a first input 71 of the code comparator 33 of the alarmmonitoring equipment 31. A second input 72 of the code comparatorcircuit 33 is provided by the reference generator 32. The reference codegenerator 32 is identical to each of the code generators associated withthe alarm source monitoring circuits 21-24, such as code generator 25associated with alarm monitoring circuit 21. The reference codegenerator comprises a four stage shift register 73 and an exclusive ORcircuit 74 having inputs 75 and 76 respectively connected to the outputsof the first and fourth stages of the shift register 73, and an output77, connected to the input of the first stage of the shift register 32.The reference code generator 32 is controlled by sync pulses provided bysync pulse generator 34 to provide the sequence of pseudo-random bitsshown in Table III that is provided by each of the code generatorsassociated with the alarm sources 11-14. It is pointed out that the syncpulse generator 34 provides gating signals for the shift registersassociated with each code generator including the reference codegenerator 32 such that each code generator provides the same outputduring a given cycle of the sync pulse generator. The sync pulsegenerator provides a sync pulse at the completion of each cycle toadvance the code generators to the next code in the sequence. Sync pulsegenerator 34 also will drive monostable multivibrator 114 (FIG. 3) witha delay to initiate the return of the code bits from the remote alarmsource locations, one at a time under the control of the multiplexercircuit 28.

The sync or interrogate pulses in addition to advancing the state of allcode generators will also advance the state of the ring counter 60 (FIG.3) of the multiplexer 28 associated with the remote alarm sources 1 1-14and a ring counter 86 (FIG. 4) of the demultiplexer circuit 35 of thealarm line monitoring circuits 31. Transmission delays anywhere in thesystem can be compensated for by adding corresponding delays in thefaster portions of the system by using a monostable multivibrator suchas multivibrator 114 in FIG. 3.

The code comparator circuit 33 may comprise a logical comparatorcircuit, such as an exclusive OR circuit 78. The exclusive OR circuit 78provides a first output level, such as a logic 0 level whenever thesignals on inputs 71 and 72 are the same, as is the case when no alarmis being transmitted from a remote alarm source. On the other hand,whenever the signals on inputs 71 and 72 are not the same, the exclusiveOR circuit 78 provides a second output, a logic 1 level which indicatesthe presence of an alarm.

The comparator circuit 33 further includes a latch circuit 79 connectedto the output of the exclusive OR circuit 78 and operable when set toinitiate a sustained alarm output in response to the detection by thecomparator circuit of either a single or a multiple code difference. Thelatch circuit 79 remains latched until a reset pulse from a voltagesource (not shown) is applied to the RESET input of the latch circuit79. To avoid nuisance alarms resulting from a single code differencecaused by noise or an inadvertent dropped bit, the latch circuit 79 maybe driven by a counter 80 which will inhibit operation of the latchcircuit 79 until several successive bits are in error.

The outputs of the code comparator circuit 33 are passed to thedemultiplexer circuit 35. The demultiplexer circuit 35 is similar to themultiplexer circuit 28 associated with the remote alarm sourcemonitoring circuits and includes a multistage ring counter 86 and aplurality of AND gates, such as gates 81-84 shown in FIG. 4. The ANDgates 81-84 drive alarm lamps 91-94, respectively, which comprise thealarm register circuit 36. A separate alarm lamp, such as lamps 91-94,is provided for each alarm source, such as alarm sources 1 l-14,respectively.

The ring counter 86 has N stages, one for each alarm source. One of thestages of the ring counter 86 contains a logic 1 level bit and theremaining stages contain logic 0 level bits. The outputs of ring counterstages, such as stages 1, 2, N-1 and N, are individually connected toone input of the AND gates 81-84, respectively. Second inputs of ANDgates 81-84 are connected together to the output of the latch circuit 79of the code comparator circuit 33.

The demultiplexer circuit 35 serves to provide an enabling signal forAND gates 81-84, in sequence, as the multiplexer 28 sequentially gatesthe outputs of the alarm source monitoring circuits 21-24 onto the alarmtransmission line 30. It is pointed out that the demultiplexer circuit35 operates in synchronism with the multiplexer circuit 28 andaccordingly, when the sync pulse line to multiplexer 28 includes adivide-by-four circuit 68 for enabling the transmission of multiple bitsfrom each alarm location, the demultiplexer sync line would include asimilar divide-by-four circuit 68'.

Thus, for example, when the logic 1 level is contained in the firststage of the ring counter 86 of the demultiplexer circuit 35, the singlelogic 1 level contained in the ring counter 60 of the multiplexer 28will also be in the first stage of ring counter 60. The logic bitcontained in the ring counter 86 of the demultiplexer circuit 35 isshifted from stage to stage of the counter (and simultaneously fromstage to stage of counter 60 of the multiplexer 28) under the control ofsync pulses provided by the system sync pulse generator 34. Therefore,assuming initially that the logic 1 level bit is stored in the firststage of the counter 86, (and counter 60) an enabling signal will beprovided to an input of AND gate 81 and thereafter in a controlledsequence to inputs of AND gates 82-84 as the logic 1 level bit isshifted through the counter 86. At the same time, initially the outputof alarm source monitoring circuit 21 will be gated onto thetransmission line 30, and then in sequence the outputs provided by alarmsource monitoring circuits 22-24.

Accordingly, whenever one of the alarm sources, such as alarm source 11,is providing an alarm indication thereby complementing the codetransmitted by the associated alarm source monitoring circuit 21, thelogic 1 output provided by the code comparator circuit 33 will be gatedthrough AND gate 81 when the gate 81 is enabled by the output of counter86 to light alarm indicator lamp 91 corresponding to alarm source 11 toregister an alarm at alarm zone one.

ALTERNATIVES An added feature of the invention is what one might callsecond level encoding. The pseudo-random sequence generator representsthe first level of coding. The second level of coding would derive fromchanges made from time to time in the feedback connections of thepseudo-random code generators such as code generators 28 and 32.

For example, the output may be taken from any stage or from severalstages this can be varied on a fixed time basis or after a preset numberof cycles. The output code can be complemented periodically. Thefeedback connections can be changed via inputs on switches, inputs readfrom punched cards, or tape or read-only memories. Thus, in FIGS. 2 and4, the outputs of the first and fourth stages of shift registers 45 and73 are shown connected over switches 147, 146, and 175, 176 to input ofcorresponding exclusive OR circuits 48 and 74. The feedback connectionscan be changed by opening switches 146, 147, 175 and 176 and closingswitches 146', 147', 175' and 176' whereby the outputs of the second andthird stages of shift registers 45 and 73 are connected to inputs ofexclusive OR circuits 48 and 74, respectively. Consequently, codegenerators 25 and 32 will each provide a further pseudo-random sequenceof bits with the bit sequence provided by code generator 25 beingidentical with the sequence provided by code generator 32.

The coded signals need not be transmitted pulses, but rather they mayappear as modulation of carrier signal such as a tone transmissionsystem. This would in no way detract from the operation of the inventionas described to this point, and it may enhance it as the tone signalswould tend to keep the line busier" from the attackers point of view; itwould thus be harder to simulate the codes. Also, note that themodulation of a carrier can be 100 percent modulation; thus tone bursts"can be used as can the switching between two tones such as is done inthe frequency shift keying systems used in telegraphy.

The preferred implementation of the code generators described in detailin this disclosure is not the only implementation possible. It does,however, represent an economical solution to the problem. In thebroadest context, any code generator capable of being duplicated andsimilarly capable of producing a large number of arbitrary codes can beused. Thus any sort of memory system, in particular a read-only-memory,which is capable of a sufficient amount of storage could be used. Also,a card reader or tape reader loaded with sufficient coded cards or tapecould be used to generate the required line security codes.

Moreover while an exemplary embodiment of the invention employsmultiplexing to permit transmission of output codes provided by thealarm source monitoring circuits of a plurality of alarm sources, it isapparent that each alarm source monitoring circuit could be connected bya separate transmission line directly to an alarm line monitoringcircuit which is individually associated with a given alarm source.

We claim:

1. In a security system including a transmission line carrying alarminformation from at least one protected area to a central monitoringarea that is remote from the protected area, means at said protectedarea for generating a monitor code comprising a selected sequence ofcode bits for transmission over the transmission line from said oneprotected area to the central monitoring area, means for modifying thecode bits provided by said generating means to indicate an alarm, meansat said central monitoring area for generating a reference codecomprised of a selected sequence of code bits that is identical to themonitor code sequence, means for comparing the monitor code with thereference code, and means for providing an alarm indication whenever thecodes are different.

2. In a transmission line security system including a transmission linefor carrying alarm information from at least one protected area to acentral monitoring area that is remote from the protected area, alarmsource means including intrusion detector means at said protected area,alarm source monitoring means at said protected area including firstcode generator means for generating a monitoring code comprising a knownpseudo-random sequence of bits for transmission to said centralmonitoring area over the transmission line at predetermined intervalsand modifying means controlled by said intrusion detector means inresponse to the detection of an unauthorized entry to modify the codebits generated by said first code generator means, and transmission linemonitoring means at said central monitoring area including second codegenerator means for generating a reference code that is identical withthe monitoring code, and code comparator means for receiving themonitoring code transmitted to said central monitoring area andcomparing the monitoring code with the reference code, and registermeans controlled by said code comparator means providing an alarmindication whenever the codes are different.

3. A transmission line security system as set forth in claim 2 whereinsaid first code generator means comprises first multistage shiftregister means, and first feed back means including exclusive OR gatingmeans connected between outputs of certain stages of said shift registermeans and the input of said shift register means.

4. A transmission line security system as set forth in claim 3 whereinsaid second code generator means includes further multistage shiftregister means and second feedback means including further exclusive ORgating means connected between outputs of certain stages of said furthershift register means and the input of said further shift register means.

5. A transmission line security system as set forth in claim 4 whereinsaid first feedback means includes means for altering the feedbackconnections of the shift register means of said first code generatormeans whereby said first code generator means provides a furtherpseudo-random sequence of bits, and wherein said second feedback meansincludes means for altering the feedback connections of the shiftregister means of said second code generator means whereby said secondcode generator means provides a pseudo-random sequence of bits that isidentical with the bit sequence provided by said first code generatormeans.

6. A transmission line security system as set forth in claim 2 includingsync pulse generating means for synchronizing the operation of saidfirst and second code generator means to provide a pseudo-randomsequence of bits at said predetermined intervals.

7. In a transmission line security system including a transmission linefor carrying alarm information from at least one protected area to acentral monitoring area that is remote from the protected area, alarmsources means including intrusion detector means at said protected area,alarm source monitoring means at said protected area including firstcode generator means for generating a monitoring code comprising a knownsequence of bits for transmission to said central monitoring area overthe transmission line at predetermined intervals and code complementingmeans controlled by said intrusion detector means in responsive to thedetection of an unauthorized entry for inverting each bit of the bitsequency generated by said first code generator means, and transmissionline monitoring means at said central monitoring area including secondcode generator means for generating a reference code that is identicalwith the monitoring code, and code comparator means for receiving themonitoring code transmitted to said central monitoring area andcomparing the monitoring code with the reference code, and registermeans controlled by said code comparator means providing an alarmindication whenever the codes are different.

8. A transmission line security system as set forth in claim 7 whereinsaid alarm source monitoring means further includes alarm convertermeans controlled by said detector means for providing first and secondlogic level signals to indicate respectively, that the protected area issecure or that an unauthorized entry has occurred, and wherein said codecomplementing means includes exclusive OR gating means having a firstinput connected to the output of said alarm converter means and a secondinput connected to the output of said first code generator means, theoutput of said code complementing means being connected to saidtransmission line whereby the code bits output at said first codegenerator means are passed over said code complementing means to saidtransmission line whenever a first logic level signal is provided bysaid alarm source converter means and the code bits output at said firstcode generator means are inverted by said code complementing means asthey are passed to said transmission line whenever said second logiclevel signal is provided by said alarm source converter means.

9. A transmission line security system as set forth in claim 7 whereinthe bits of at least two successive pseudo-random bit sequences areinverted by said code complementing means whenever an unauthorized entryis detected.

10. A transmission line security system as set forth in claim 7 whereinsaid code comparator means includes gating means for comparing each bitof the monitoring code received from said one protected area, with eachbit of the reference code and providing an enabling signal whenever oneor more of the bits of the monitoring code differs with the bits of thereference code and latch means responsive to said enabling signal toprovide an alarm output representing an alarm condition said registermeans being controlled by said alarm output to register said alarm.

11. A transmission line security system as set forth in claim 10 whereinsaid code comparator means includes counter means for providing afurther enabling signal for said latch means whereby said latch meansprovides an alarm output only when two or more successive bits of themonitoring code sequence differ with corresponding bits of the referencecode sequence.

12. A transmission line security system as set forth in claim 10 whereinsaid code comparator means comprises exclusive OR gating means having afirst input connected to said transmission line for receiving themonitoring code transmitted from said protected area and a second inputconnected to the output of said second code generator means forreceiving the reference code, said exclusive OR gating means providing afirst logic level output wherever the code bits compared are the sameand a second logic level output for enabling said latch means wheneverthe code bits compared are different.

13. In a transmission line security system including a transmission linefor carrying signals from a plurality of protected areas to a centralmonitoring area that is remote from said protected areas, each of saidprotected areas having individual monitor code generator means forgenerating a line monitoring code comprising a selected sequence of codebits, switching means for sequentially connecting the output of each ofsaid monitor code generator means to said transmission line to therebypass a different bit of the sequence of bits provided by each monitorcode generator means to said transmission line during successive timeintervals whereby a further selected sequence of bits is transmitted tosaid central monitoring area over said transmission line with successivebits of the further bit sequence being provided by different ones ofsaid monitor code generator means, reference code generators means atsaid central monitoring area for generating a reference code comprisinga selected sequence of code bits, each bit of which is normallyidentical with corresponding bits of the further bit sequence at anygiven time, and means for comparing the further bit sequence with thereference bit sequence and providing an indication whenever any of thecorresponding bits of the two sequences are different.

14. A transmission line security system as set forth in claim 13 whereinsaid code generator means all generate a known pseudorandom sequence ofcode bits and wherein the further bit sequence provided by saidswitching means is identical to the pseudo-random sequence of bitsprovided by each of said monitor code generator means.

15. In a transmission line security system including a transmission linefor carrying alarm information from a plurality of protected areas to acentral monitoring area that is remote from the protected area, each ofsaid protected areas having individual alarm indicating means, monitorcode generator means for generating a known sequence of coded bits, andmeans controlled by said alarm indicating means to modify the code bitsgenerated by an associated monitor code generator means, multiplexermeans for sequentially connecting the output of the monitor codegenerator means at each of said protected areas to the transmission lineto enable the transmission to said central monitoring area of adifferent one or more of the bits provided by each of said monitor codegenerator means thereby providing a further sequence of coded bits, andtransmission line monitoring means at said central monitoring areaincluding reference code generator means for generating a sequence ofcoded bits that is identical with the further bit sequence provided bysaid multiplexer means, comparator means for receiving the further bitsequence and comparing each bit of the further bit sequence with thecorresponding bit of the reference bit sequence, and alarm registeringmeans controlled by said comparator means for registering an alarmwhenever one or more of the bits of the further bit sequence differswith corresponding bits of the reference bit sequence.

16. A transmission line security system as set forth in claim whereinsaid alarm registering means includes demultiplexer means controlled bysaid comparator means whenever one or more of the bits of the furtherbit sequence differs with corresponding bits of the reference bitsequence to identify each bit of the further bit sequence which differswith a corresponding bit of the reference bit sequence.

17. A transmission line security system as set forth in claim 16 whereinsaid multiplexer means comprises a plurality of gate circuits, eachhaving a first input individually connected to an output of the codegenerator means of one of said protected areas, and means forselectively enabling the gate circuits to pass at least one bit of thesequence generated by an associated code generator means to thetransmission line.

18. A transmission line security system as set forth in claim 17 whereinsaid demultiplexer means comprises a further plurality of gate circuitsincluding an individual demultiplexer gate for each protected area, saidde multiplexer gates having first inputs connected together to theoutput of said comparator means and means for sequentially enabling eachof said demultiplexer gates whereby each demultiplexer gate provides afirst logic level output whenever the compared bits are the same, and asecond logic level output whenever the compared bits are different.

19. A transmission line security system as set forth in claim 18 whereinsaid alarm registering means further includes a separate indicator foreach of said protected areas, each indicator being individuallyconnected to the output of one of said demultiplexer gates to beenergized whenever the corresponding demultiplexer gate provides saidsecond logic level output.

20. A transmission line security system as set forth in claim 18 whereinsaid multiplexer enabling means includes multi-stage ring counter meanshaving a separate stage for each protected area and an enabling signalstored in one of the stages of said multi-stage ring counter means, eachof said multiplexer gate circuits having a second input connected to theoutput of a corresponding one of the stages of said ring counter means,and wherein said demultiplexer enabling means includes furthermulti-stage ring counter means having a separate stage for eachprotected area and an enabling signal stored in one of the stages ofsaid further multistage ring counter means, each of said further gatecircuits having a second input connected to the output of acorresponding one of the stages of said further ring counter means, andsync pulse generating means for providing sync pulses for the ringcounter means of the multiplexer and demultiplexer means for shiftingsaid enabling signal from stage to stage of said ring counter meanswhereby said enabling signals are selectively applied to the secondinputs of said gate circuits in sequence.

21. A transmission line security system as set forth in claim 20 whereinthe monitor code generator means associated with said protected areasand said reference code generator means each include multi-stage shiftregister means and associated feedback means for selectively connectingoutputs of certain stages of said shift register means to thecorresponding input of said shift register means, said sync pulses beingextended to the shift register means of said monitor and reference codegenerator means to shift the signals output from associated gating meansthrough the shift register means of said code generator means wherebysaid sequence of coded bits is provided at the output of the shiftregister means of said monitor and reference code generator means.

22. A transmission line security system as set forth in claim 20 whereinthe monitor code generator means associated with said protected areasand said reference code generator means each include multi-stage shiftregister means and associated feedback means for selectively connectingthe outputs of certain stages of said shift register means to thecorresponding input of said shift register means of said monitor andreference code generator means to shift the signals output from anassociated gating means through the shift register means of said codegenerator means whereby said sequence of coded bits is provided at theoutput of the shift register means of said monitor and reference codegenerator means, said multiplexer means and said demultiplexer meanseach including pulse divider means for dividing the sync pulse rate by apredetermined number of the coded bits of the sequence provided by eachmonitor code generator means and sequentially transmitted over saidtransmission line to said central monitoring area and compared with acorresponding number of bits provided by the reference code generatormeans. 1

1. In a security system including a transmission line carrying alarminformation from at least one protected area to a central monitoringarea that is remote froM the protected area, means at said protectedarea for generating a monitor code comprising a selected sequence ofcode bits for transmission over the transmission line from said oneprotected area to the central monitoring area, means for modifying thecode bits provided by said generating means to indicate an alarm, meansat said central monitoring area for generating a reference codecomprised of a selected sequence of code bits that is identical to themonitor code sequence, means for comparing the monitor code with thereference code, and means for providing an alarm indication whenever thecodes are different.
 2. In a transmission line security system includinga transmission line for carrying alarm information from at least oneprotected area to a central monitoring area that is remote from theprotected area, alarm source means including intrusion detector means atsaid protected area, alarm source monitoring means at said protectedarea including first code generator means for generating a monitoringcode comprising a known pseudo-random sequence of bits for transmissionto said central monitoring area over the transmission line atpredetermined intervals and modifying means controlled by said intrusiondetector means in response to the detection of an unauthorized entry tomodify the code bits generated by said first code generator means, andtransmission line monitoring means at said central monitoring areaincluding second code generator means for generating a reference codethat is identical with the monitoring code, and code comparator meansfor receiving the monitoring code transmitted to said central monitoringarea and comparing the monitoring code with the reference code, andregister means controlled by said code comparator means providing analarm indication whenever the codes are different.
 3. A transmissionline security system as set forth in claim 2 wherein said first codegenerator means comprises first multistage shift register means, andfirst feed back means including exclusive OR gating means connectedbetween outputs of certain stages of said shift register means and theinput of said shift register means.
 4. A transmission line securitysystem as set forth in claim 3 wherein said second code generator meansincludes further multistage shift register means and second feedbackmeans including further exclusive OR gating means connected betweenoutputs of certain stages of said further shift register means and theinput of said further shift register means.
 5. A transmission linesecurity system as set forth in claim 4 wherein said first feedbackmeans includes means for altering the feedback connections of the shiftregister means of said first code generator means whereby said firstcode generator means provides a further pseudo-random sequence of bits,and wherein said second feedback means includes means for altering thefeedback connections of the shift register means of said second codegenerator means whereby said second code generator means provides apseudo-random sequence of bits that is identical with the bit sequenceprovided by said first code generator means.
 6. A transmission linesecurity system as set forth in claim 2 including sync pulse generatingmeans for synchronizing the operation of said first and second codegenerator means to provide a pseudo-random sequence of bits at saidpredetermined intervals.
 7. In a transmission line security systemincluding a transmission line for carrying alarm information from atleast one protected area to a central monitoring area that is remotefrom the protected area, alarm sources means including intrusiondetector means at said protected area, alarm source monitoring means atsaid protected area including first code generator means for generatinga monitoring code comprising a known sequence of bits for transmissionto said central monitoring area over the transmission line atpredetermined intervals and code complementing means controlled by saidintrusion detector Means in responsive to the detection of anunauthorized entry for inverting each bit of the bit sequency generatedby said first code generator means, and transmission line monitoringmeans at said central monitoring area including second code generatormeans for generating a reference code that is identical with themonitoring code, and code comparator means for receiving the monitoringcode transmitted to said central monitoring area and comparing themonitoring code with the reference code, and register means controlledby said code comparator means providing an alarm indication whenever thecodes are different.
 8. A transmission line security system as set forthin claim 7 wherein said alarm source monitoring means further includesalarm converter means controlled by said detector means for providingfirst and second logic level signals to indicate respectively, that theprotected area is secure or that an unauthorized entry has occurred, andwherein said code complementing means includes exclusive OR gating meanshaving a first input connected to the output of said alarm convertermeans and a second input connected to the output of said first codegenerator means, the output of said code complementing means beingconnected to said transmission line whereby the code bits output at saidfirst code generator means are passed over said code complementing meansto said transmission line whenever a first logic level signal isprovided by said alarm source converter means and the code bits outputat said first code generator means are inverted by said codecomplementing means as they are passed to said transmission linewhenever said second logic level signal is provided by said alarm sourceconverter means.
 9. A transmission line security system as set forth inclaim 7 wherein the bits of at least two successive pseudo-random bitsequences are inverted by said code complementing means whenever anunauthorized entry is detected.
 10. A transmission line security systemas set forth in claim 7 wherein said code comparator means includesgating means for comparing each bit of the monitoring code received fromsaid one protected area, with each bit of the reference code andproviding an enabling signal whenever one or more of the bits of themonitoring code differs with the bits of the reference code and latchmeans responsive to said enabling signal to provide an alarm outputrepresenting an alarm condition said register means being controlled bysaid alarm output to register said alarm.
 11. A transmission linesecurity system as set forth in claim 10 wherein said code comparatormeans includes counter means for providing a further enabling signal forsaid latch means whereby said latch means provides an alarm output onlywhen two or more successive bits of the monitoring code sequence differwith corresponding bits of the reference code sequence.
 12. Atransmission line security system as set forth in claim 10 wherein saidcode comparator means comprises exclusive OR gating means having a firstinput connected to said transmission line for receiving the monitoringcode transmitted from said protected area and a second input connectedto the output of said second code generator means for receiving thereference code, said exclusive OR gating means providing a first logiclevel output wherever the code bits compared are the same and a secondlogic level output for enabling said latch means whenever the code bitscompared are different.
 13. In a transmission line security systemincluding a transmission line for carrying signals from a plurality ofprotected areas to a central monitoring area that is remote from saidprotected areas, each of said protected areas having individual monitorcode generator means for generating a line monitoring code comprising aselected sequence of code bits, switching means for sequentiallyconnecting the output of each of said monitor code generator means tosaid transmission line to thereby pass a different bit of the sequenCeof bits provided by each monitor code generator means to saidtransmission line during successive time intervals whereby a furtherselected sequence of bits is transmitted to said central monitoring areaover said transmission line with successive bits of the further bitsequence being provided by different ones of said monitor code generatormeans, reference code generators means at said central monitoring areafor generating a reference code comprising a selected sequence of codebits, each bit of which is normally identical with corresponding bits ofthe further bit sequence at any given time, and means for comparing thefurther bit sequence with the reference bit sequence and providing anindication whenever any of the corresponding bits of the two sequencesare different.
 14. A transmission line security system as set forth inclaim 13 wherein said code generator means all generate a knownpseudorandom sequence of code bits and wherein the further bit sequenceprovided by said switching means is identical to the pseudo-randomsequence of bits provided by each of said monitor code generator means.15. In a transmission line security system including a transmission linefor carrying alarm information from a plurality of protected areas to acentral monitoring area that is remote from the protected area, each ofsaid protected areas having individual alarm indicating means, monitorcode generator means for generating a known sequence of coded bits, andmeans controlled by said alarm indicating means to modify the code bitsgenerated by an associated monitor code generator means, multiplexermeans for sequentially connecting the output of the monitor codegenerator means at each of said protected areas to the transmission lineto enable the transmission to said central monitoring area of adifferent one or more of the bits provided by each of said monitor codegenerator means thereby providing a further sequence of coded bits, andtransmission line monitoring means at said central monitoring areaincluding reference code generator means for generating a sequence ofcoded bits that is identical with the further bit sequence provided bysaid multiplexer means, comparator means for receiving the further bitsequence and comparing each bit of the further bit sequence with thecorresponding bit of the reference bit sequence, and alarm registeringmeans controlled by said comparator means for registering an alarmwhenever one or more of the bits of the further bit sequence differswith corresponding bits of the reference bit sequence.
 16. Atransmission line security system as set forth in claim 15 wherein saidalarm registering means includes demultiplexer means controlled by saidcomparator means whenever one or more of the bits of the further bitsequence differs with corresponding bits of the reference bit sequenceto identify each bit of the further bit sequence which differs with acorresponding bit of the reference bit sequence.
 17. A transmission linesecurity system as set forth in claim 16 wherein said multiplexer meanscomprises a plurality of gate circuits, each having a first inputindividually connected to an output of the code generator means of oneof said protected areas, and means for selectively enabling the gatecircuits to pass at least one bit of the sequence generated by anassociated code generator means to the transmission line.
 18. Atransmission line security system as set forth in claim 17 wherein saiddemultiplexer means comprises a further plurality of gate circuitsincluding an individual demultiplexer gate for each protected area, saiddemultiplexer gates having first inputs connected together to the outputof said comparator means and means for sequentially enabling each ofsaid demultiplexer gates whereby each demultiplexer gate provides afirst logic level output whenever the compared bits are the same, and asecond logic level output whenever the compared bits are different. 19.A transmission line security system as set foRth in claim 18 whereinsaid alarm registering means further includes a separate indicator foreach of said protected areas, each indicator being individuallyconnected to the output of one of said demultiplexer gates to beenergized whenever the corresponding demultiplexer gate provides saidsecond logic level output.
 20. A transmission line security system asset forth in claim 18 wherein said multiplexer enabling means includesmulti-stage ring counter means having a separate stage for eachprotected area and an enabling signal stored in one of the stages ofsaid multi-stage ring counter means, each of said multiplexer gatecircuits having a second input connected to the output of acorresponding one of the stages of said ring counter means, and whereinsaid demultiplexer enabling means includes further multi-stage ringcounter means having a separate stage for each protected area and anenabling signal stored in one of the stages of said further multistagering counter means, each of said further gate circuits having a secondinput connected to the output of a corresponding one of the stages ofsaid further ring counter means, and sync pulse generating means forproviding sync pulses for the ring counter means of the multiplexer anddemultiplexer means for shifting said enabling signal from stage tostage of said ring counter means whereby said enabling signals areselectively applied to the second inputs of said gate circuits insequence.
 21. A transmission line security system as set forth in claim20 wherein the monitor code generator means associated with saidprotected areas and said reference code generator means each includemulti-stage shift register means and associated feedback means forselectively connecting outputs of certain stages of said shift registermeans to the corresponding input of said shift register means, said syncpulses being extended to the shift register means of said monitor andreference code generator means to shift the signals output fromassociated gating means through the shift register means of said codegenerator means whereby said sequence of coded bits is provided at theoutput of the shift register means of said monitor and reference codegenerator means.
 22. A transmission line security system as set forth inclaim 20 wherein the monitor code generator means associated with saidprotected areas and said reference code generator means each includemulti-stage shift register means and associated feedback means forselectively connecting the outputs of certain stages of said shiftregister means to the corresponding input of said shift register meansof said monitor and reference code generator means to shift the signalsoutput from an associated gating means through the shift register meansof said code generator means whereby said sequence of coded bits isprovided at the output of the shift register means of said monitor andreference code generator means, said multiplexer means and saiddemultiplexer means each including pulse divider means for dividing thesync pulse rate by a predetermined number of the coded bits of thesequence provided by each monitor code generator means and sequentiallytransmitted over said transmission line to said central monitoring areaand compared with a corresponding number of bits provided by thereference code generator means.